Plastik dişli çarkların konstrüktif özelliklerinin incelenmesi

Abstract

ÖZET Plastik malzemeler tüm dünyada gerek araştırma ve geliştirme, gerekse tatbikat alanı bakımından hızlı gelişme göstermektedir. Hafif, kolay islenebilir, korozyona karşı mukavemetli, iyi elektrik ve ısı yalıtkanlığına, iyi bir yüzey kalitesine ve görünüşe sahip plastikler makina, uçak, elektronik ve ev aletleri gibi sanayinin hemem hemen tüm dallarında gün geçtikçe daha çok kullanılmaktadır. Plastikler, dişli çark malzemeleri olarak üstün özellikleri nedeniyle tercih edilmektedir. Plastik dişli çarklar sessiz çalışma, yüksek aşınma mukavemeti, düşük sürtünme katsayıları gibi üstün özelliklere haizdirler. Bir çok tatbikat alanında metallere rakip olan plastik malzemeler saatlerin hassas dişli çarklarından en korozif ortamlarda çalışan mekanizmalara kadar kullanılmaktadır. Plastik malzemelerin genel özelliklerinin incelenmesinden sonra en çok kullanılan dişli malzemelerinin özellikleri verilmiştir. 3. Bölümde alın dişli çarkların dizayn esasları incelenmiştir. Dizaynda esas olan hesaplamalar İle birlikte konstrüktif tedbirler de ele alınmıştır. 4. Böl Umde sonsuz vida mekanizmalarının dizayn esasları incelenmiştir. VII

ANALYSIS OF THE DESIGN CHARACTERISTICS OF THE PLASTIC GEARS SUMMARY Plastics are the wondering materials of the modern age» They have performance possibilities matched by nothing else, but to obtain them may require a special design» The properties of plastics differ from the traditional engineering materials» The effects of their natural limitations should be minimized» Good design and successful utilisation of plastics introduce some considerations not ordinarily of concern with other types of materials. New factors enter, and these have tremendous influence on the best engineering compromise of a group of conflicting demands. Some of the problems involved in the selection of suitable plastic and in the design of plastic parts, can be visualized when the general advantages and disadvantages are considered s Advantages - Ease of fabrication» Plastics are readily adaptable to mass production methods, and intricate parts are often made of plastic, and produced in large quantity at low unit cost. - Weight saving» In general, the density of plastics is less then that of the metals» - Resistance to corrosion» Most plastics are immune to rust, hot, or corrosion» - Electrical insulation. Distributor parts, for example, are made of plastics. - Thermal insulation. Plastics are poor heat conductors» Control handling and steering wheels utilize this property» - Transparent parts, and parts of an unlimited color range, may be made of plastics» - Damping characteristics. High damping capacity of plastics means that they damp out vibrations. Silent timing gears utilize this characteristic» Disadvantages VIII- Low strenght. Plastics as a rule are not very strong. - Dimensional ly unstable. Plastics may warp, shrink, and be subject to creep and cold flow. Also they are relatively soft and are easily scratched» - Subject to deterioration. Plastic may deteriorate on exposure to air or sunlight. Many are attacked by mildew or fungus., some even by rats and mice. - Odor. Some plastic materials have definite odors, not all of them pleasant. - Cost. Plastic materials are not cheap, and satisfactory plastic parts may be quite expensive, particulary in small lots. Fillers and reinforcing materials are used to increase the mechanical properties and to minimize the effects of natural limitations. The word modification means is to change properties of a plastic material with adding fillers and fibers into the material by special methods. Plastics are now used in almost every application, ranging from household articles to space travels from transportation to packaging 5 from medicine to toys, from bridge building to sports. Designing plastics parts for this wide range of uses has become a major activity for designers, architects, engineers, and others who are concerned with product developments. This new responsi bility for plastics oriented design in the production of technical parts confronts the designer with increasing reqirements regarding knowledge of materials and expertise in moulding operations and mould design. In the first part of the study, the definitions related to the plastics are given. First of all, it will be better to explain the meaning of the word plastic. The word plastic has been adapted as a general name for a group of materials because it means `capable of being molded`. The word however is used rather loosely to describe not only a material which can be molded but also that material after it has been fabricated into an object even though it may no longer be moldable. A plastic can be regarded as a material containing a synthetic high polymer as the major constituent. Polymers are materials of high molecular weight formed by joining together many (poly) small molecules. In some plastics the polymer may be the only constituent ( resin }, but more usually a variety of other substances are incorporated such as fillers, pigments, plasticisiers etc. Polymers are classified depending upon various criteria. IX-According to their chemical compositions polymers are divided into two groups; organic and inorgonic polymers. Organic compounds, by definition, contain carbon at the main molecular chain. Inorganic compounds contain elements like germenium at the main moleculer chain. Polymers are divided as homopolymers and copolymers according to their structure. If the polymer is formed by the repetation of the same unit then it is died as homopolymer. If it is formed by the repetation of two different units then it is called as copolymer. Fundemantally, polymers are classified depending upon the geometrical form of the polymer molecule. The first group are the linear polymers which are the basis of thermoplastics; the second group the network polymers which are the basis of thermosetting plastics. Additives are used to modify the optical, mechanical, or surface propeties of the plastic. Or they may be used simply as fillers to reduce the overall material cost. In general, additives may be classified into two categories -those which modify properties by physical means and those which achieve their effect by chemical means. There may be different grades of a plastic. These grades are; basic grades, grades with improved sliding properties, reinforced grades and impact modified grades. In the second part of the study, properties of the plastic gears are given. Plastics are used by the gearing industry in increasing amounts as the economic and technical advantages of non- metal ic gears become better appreciated. Among the advantages of plastic gears ares - an ability to operate with minimum or no lubrication, - high resiliance and internal damping capacity, resulting in quiet operation, - good corrosion and chemical resistance, - 1 ow densi ty, - elimination of machining operatins using injection moulding giving cost effectiveness. Their use in many applications is limited due to s - low load- carrying capacity compared to metal gears, - limitations on operating temperature and- low dimensional stability due to shrinkage, thermal expansion and moisture absorbtion, particulary for nylons» Inspite of these advantages. plastics are finding applications in the heavy duty range of gearing and large gears can be produced, e.g a 4.3 meter referance diameter spur gear has been manufactured from MonoCast nylon segments. At the opposite end miniature gears of only 0.75 mm referance diameter and 0.01 mm facewidth have been formed » Currently. the most common gear materials are nylons, asetal resins. polycarbonates and polyesters. Plastic gears can be made by machining from solid, casting and blanking from sheets or, for large- scale production, by injection moulding. At the table, Some of the plastic gear materials and their registered names are given. Registered Trademark of Some Materials In the third part of the study, spur gears made from plastic materials are examined. Thermoplastic gearwheels have found wide applications, particularly in precision drive systems for watches, time switches, meters etc. which are produced in large numbers. In such applications, the low production cost made possible by injection moulding is a decisive factor in the choice of plastics. A more demanding application is gearing for small electrical appliances such as food processors. In additon to low cost production, a compelling argument for the use of engineering thermoplastic is their good sliding properties which make for silent running and maintenance free operation. Thermoplastic polyester grades and asetal copolymer grades offer a wide variety of possibilities for meeting the requirements of XIdifferent gears. Different basic profiles are compared» The advantages and disadvantages of each profile are given» In power gears, only involute teeth are used and so following designs are restricted to these» Depending upon the gear requirements and operating conditions., the most desirable material properties are given in this part. The combination of materials used has an important effect on the amount of friction. The properties that the combination of materials should ensure are also given» Spur gears with plastic gearwheels are only rarely provided with splush lubrication because the cost of sealing the gear housing is high» The normal procedure is to give the gears a once-only grease lubrication during assembly» Desing calculations and other subjects which should be taken into consideration are also given» In the fourth part of the study» worm gears with worm wheels made from thermoplastic materials are examined» Worm gears are characterised by silent running, high transmission ratio in one step and relatively compact» lightweight design» This has made them the preffered type of drive for small electrical appliances such as food slicers, flour mills» etc» Worm gears may be called on to meet widely varying requirements with regard to speed ratio, transmission output» service temperatures and service life» The suitable materials and material combinations are given» Depending upon the production process and position of the shaping surfaces of the mould, the worm teeth can have different flank shapes» Types of worm and worm wheels are given» In the design of worm wheels» the important stress characteristics are studied» Factors that effect the calculations» figures and tables are given» In the fifth part of the study» the production methods of the thermoplastics are given» From the production point of view the main advantage of plastic materials is their relatively low melting point and their ability to flow into a mould» Generally there is only one production operation required to convert the raw material into its final shape» The production methods are divided into two groups - The first one is the moulding of thermoplastics and the second one is the machining of thermoplastics» Some of the methods of each group are given. The advantages and disadvantages of each method are given. XIIAs a material plastics have natural limitations. But these limitations are balanced with special methods. Plastic gears provides the most desirable properties for applications. They can be produced in many numbers with low production cost by enjection moulding. For the production of large gears Cast method is preferred. XIII